The present invention relates to a semiconductor exposing system, and more particularly to an illumination apparatus for exposure which includes an excimer laser and is suited to form a high-resolution image of a fine pattern.
As semiconductor devices are made high in integration density and have a fine circuit pattern, attention is paid to the use of a laser capable of emitting a pulsed light beam of far ultraviolet region such as an excimer laser, as a light source for exposure. Such a light source is disclosed in a Japanese Patent application JP-A No. 57-198,631. The laser beam emitted from an injection locked excimer laser is high in energy density and narrow in spectral width. Further, this laser beam has a single transverse mode and high spatial coherency.
Accordingly, a wavefront distorted irregularly by the imperfections of various optical elements will produce an irregular interference pattern (which is called "speckle") on a projected image. Two methods for eliminating the above speckle have been proposed. In one of the methods, as described in Japanese Patent Applications JP-A No. 60-230,629 and JP-A No. 60-247,643, an optical member for generating an optical path difference greater than coherent length is disposed on an optical path. In the other method, as described on page 35 of the SPIE, Vol. 174, "Development in Semiconductor Microlithography" IV, 1979, a laser beam is deflected successively by means of an optical deflector, and a plurality of laser pulses which are different in deflection angle from one another (directional light pulses) are successively impinged upon a receiving surface to superpose plural different speckles, and consequently, the uniformity of illumination on the receiving surface is improved In the former method, the optical member for generating an optical path difference is formed of a bundle of a plurality of square pillar prisms or optical fibers different in length. The laser beam emitted from an injection locked excimer laser has a coherent length of about 30 cm. In order to make the optical path difference between adjacent two of optical elements used more than 30 cm, the optical member for generating an optical path difference is required to be very large in size. On the other hand, the latter method has an advantage that an optical system used is relatively compact. In the latter method, however, no regard has been paid to the degree of contribution of each of a plurality of laser pulses which are used for one exposure operation, to the reaction of a light sensitive material with light.
The light sensitive material concentration which has not yet reacted with light, varies with exposure to light in accordance with the following equation: ##EQU1## where M indicates the concentration which has not yet reacted with light, t an exposure time, k a constant, and I the intensity of light.
In a case where the light intensity I is kept constant, the equation (1) is rewritten as follows: ##EQU2## where C is a constant.
FIG. 5 is a graph showing an M-t curve which is obtained from the equation (2). Referring to FIG. 5, the exposure time elapses in order of t.sub.1, t.sub.2, t.sub.3 and t.sub.4, and a time interval between the time moments t.sub.1 and t.sub.2 is made equal to a time interval between the time moments t.sub.3 and t.sub.4. As is apparent from FIG. 5, a change in concentration M in the time interval between t.sub.3 and t.sub.4 is far smaller than a change in concentration M in the time interval between t.sub.1 and t.sub.2, that is, the reaction of the light sensitive material with light is weakened with exposure time elapsing. Accordingly, in a case where a plurality of light pulses are used in a single exposure operation, the picture quality of an image formed on a light receiving surface is greatly affected by speckles due to several initial directional light pulses.
That is, in the latter method which uses a plurality of light pulses for a single exposure operation, no regard has been paid to a fact that the reaction of the light sensitive material with a light pulse weakens as the light pulse is received later, and thus there arises a problem that a developed pattern on a semiconductor wafer is strongly affected by speckles due to several initial directional light pulses